mkinitfs: Introduce deviceinfo_append_dtb variable (!387)

We don't need to append the dtb to the kernel image in all cases, with
e.g. the u-boot bootloader we can load the dtb seperately from the
kernel image. Introduce a new variable deviceinfo_append_dtb, if set to
"true", append the dtb, otherwise just copy the dtb file to the boot
partition.

Fixes #260

[ci:ignore-count]